Effect of mixed partial occupation of metal sites on the phase stability of γ-Cr_{23- x} Fe _x C₆ (x = 0-3) carbides

The effect of mixed partial occupation of metal sites on the phase stability of the γ-Cr_23-x Fe _x C₆ (x = 0-3) carbides is explored as function of composition and temperature. Ab initio calculations combined with statistical thermodynamics approaches reveal that the site occupation of the carbides may be incorrectly predicted when only the commonly used approach of full sublattice occupation is considered. We found that the γ-M₂₃C₆ structure can be understood as a familiar sodium chloride structure with positively charged rhombic dodecahedron (M^(4a) M₁₂ ^(48h)) and negatively charged cubo-octahedron (M₈ ^(32f) C₆ ^(24e)) super-ion clusters, together with interstitial metal atoms at the 8c sites. The stability of the partially occupied phase can be easily rationalized on the basis of a super-ion analysis of the carbide phase. This new understanding of γ-M₂₃C₆ carbides may facilitate further development of high-chromium heat-resistant steels.